Possible solution. Revert to CWM 4.x, revert to your older backup, update to CWM 5.x, create new backup. Can CWM5.x restore it then? If so, then this kind of jerry-rig update should be done for all backups that people have made before making the permanent switch to 5.x. This is what I plan to do, but I'm not gonna be in a good place to flash for a little while.
If anyone can confirm this solution as working, I'll get a separate thread going as a [How-to] for updating to CWM 5.x. PMs are fine, replys to this message are fine too. Keep me posted.
All the best,
-HG